The sea gives off again by evaporation as much water as it receives from rain and from the rivers of the land. But the salt carried into it remains behind. If you take some salt water and evaporate it the pure water disappears, and the salt is left. So it is with the sea. Streams are every day carrying fresh supplies of salt into the sea. Every day, too, millions of tons of water are passing from the ocean into vapor in the atmosphere. The waters of the sea must consequently be getting salter by degrees. The process, however, is an extremely slow one.

Although sea water has probably been gradually growing in saltness ever since rivers first flowed into the great sea, it is even now by no means as salt as it might be. In the Atlantic Ocean, for example, the total quantity of the different salts amounts only to about three and a half parts in every hundred parts of water. But in the Dead Sea, which is extremely salt, the proportion is as much as twenty-four parts in the hundred of water.

Standing by the shore and watching for a little the surface of the sea, you notice how restless it is. Even on the calmest summer day, a slight ripple or a gentle heaving motion will be seen.

Again, if you watch a little longer, you will find that whether the sea is calm or rough, it does not remain always at the same limit upon the beach. At one part of the day the edge of the water reaches to the upper part of the sloping beach; some six hours afterward it has retired to the lower part. You may watch it falling and rising day by day, and year by year, with so much regularity that its motion can be predicted long beforehand. This ebb and flow of the sea forms what are called tides.

If you cork up an empty bottle and throw it into the sea, it will of course float. But it will not remain long where it fell. It will begin to move away, and may travel for a long distance until thrown upon some shore again. Bottles cast upon mid-ocean have been known to be carried in this way for many hundreds of miles. This surface-drift of the sea water corresponds generally with the direction in which the prevalent winds blow.

But it is not merely the surface water which moves. You have learnt a little about icebergs; and one fact about them which you must remember is that, large as they may seem, there is about seven times more of their mass below water than above it. Now, it sometimes happens that an iceberg is seen sailing on, even right in the face of a strong wind. This shows that it is moving, not with the wind, but with a strong under-current in the sea. In short, the sea is found to be traversed by many currents, some flowing from cold to warm regions, and others from warm to cold.

Here, then, are four facts about the sea:—1st, it has a restless surface, disturbed by ripples and waves; 2ndly, it is constantly heaving with the ebb and flow of the tides; 3dly, its surface waters drift with the wind; and 4thly, it possesses currents like the atmosphere.

For the present it will be enough if we learn something regarding the first of these facts—the waves of the sea.

Here again you may profitably illustrate by familiar objects what goes on upon so vast a scale in nature. Take a basin, or a long trough of water, and blow upon the water at one edge. You throw its surface into ripples, which, as you will observe, start from the place where your breath first hits the water, and roll onward until they break in little wavelets upon the opposite margin of the basin.

What you do in a small way is the same action by which the waves of the sea are formed. All these disturbances of the smoothness of the sea are due to disturbances of the air. Wind acts upon the water of the sea as your breath does on that of the basin. Striking the surface it throws the water into ripples or undulations, and in continuing to blow along the surface it gives these additional force, until driven on by a furious gale they grow into huge billows.